This invention relates to a motor control system with a current source power converter, and more particularly to a method and open loop control circuit for modulating the input current to a controlled current inverter to reduce the harmonic current content and torque pulsations associated with these drive systems.
Adjustable speed ac motor drives using the current source inverter approach are operative to supply rectangular or flat-topped currents to the stator windings of the motor being energized. Assuming that the current source or controlled current inverter is a three-phase full wave bridge type inverter, torque pulsations at the sixth harmonic of the motor frequency caused by the non-sinusoidal current waveform can occur. The sixth harmonic torque pulsations result from interaction of the fifth and seventh current harmonics with the fundamental frequency, and are especially troublesome at very low motor speeds. During starting or when passing through zero speed to reverse the direction of rotation, the torque pulsations result in instabilities and uneven running.
A closed loop technique for reducing torque ripple which requires the continuous calculation of torque from sensed motor parameters is disclosed in U.S. Pat. No. 3,919,609 to Klautschek et al. The sensed torque is compared to a reference value and the difference used to modulate the input current. Also, there are various prior art techniques such as pulse width modulation that could be considered to reduce the harmonic content of the output current waveform. However, pulse width modulation approaches require higher cost main SCR's and a more complex control. The present invention is directed to an input current modulation technique which requires no additional power components and only a small addition to the control circuit, does not require measurement of actual torque, and is most effective at very low motor speeds where it is most required.